The quality and performance of a digital transmitter is determined by the distance over which the transmitted digital signal can propagate without severe distortions. This is typically characterized as the distance over which a dispersion penalty reaches a level of ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated source, can transmit up to a distance of ˜50 km in standard single mode fiber at 1550 nm before the dispersion penalty reaches the level of ˜1 dB. This distance is typically called the dispersion limit. The dispersion limit is determined by the fundamental assumption that the digital signal is transform-limited, i.e., the signal has no time-varying phase across its bits and the signal has a bit period of 100 ps, or 1/(bit rate) for a 10 Gb/s.
In pending prior U.S. patent application Ser. No. 11/068,032, filed Feb. 28, 2005 by Daniel Mahgerefteh et al. for OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT and/or pending prior U.S. Provisional Patent Application Ser. No. 60/566,060, filed Apr. 28, 2004 by Daniel Mahgerefteh et al. for METHOD OF TRANSMISSION USING PARTIAL FM AND AM MODULATION, which patent applications are hereby incorporated herein by reference, there is disclosed a method for generation of an optical signal with increased dispersion tolerance based on FM modulation of an AM signal. A preferred embodiment of the invention is a novel transmitter comprising a directly modulated laser followed by an optical spectrum reshaper (OSR). This transmitter is sometimes referred to as a Chirp Managed Laser (CML™) by Azna LLC of Wilmington Mass. An important aspect of the CML™ is the generation of an AM signal having a flat-topped instantaneous frequency profile having fast rise times and fall times relative to the intensity profile of the signal.